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Stephen J. Lord

Abstract

The verification of the Arakawa and Schubert (1974) cumulus parameterization is continued using a semi-prognostic approach. Observed data from Phase III of GATE are used to provide estimates of the large-scale forcing of a cumulus ensemble at each observation time. Instantaneous values of the precipitation and the warming and drying due to cumulus convection are calculated using the parameterization.

The results show that the calculated precipitation agrees very well with estimates from the observed large-scale moisture budget and from radar observations. The calculated vertical profiles of cumulus warming and drying also are quite similar to the observed. It is shown that the closure assumption adopted in the, parameterization (the cloud-work function quasi-equilibrium) results in errors of generally <10% in the calculated precipitation. The sensitivity of the parameterization to some assumptions of the cloud ensemble model and the solution method for the cloud-base mass flux is investigated.

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Stephen J. Lord and Jacqueline M. Lord

Abstract

A statistical analysis of several experiments with different microphysical parameterizations in an axisymmetric, nonhydrostatic tropical cyclone model illustrates the impact of icc-phase microphysics on model vertical velocity structure. The parameterizations are designed to illustrate the effects of 1) thermodynamic input through latent heating, 2) vertical sorting of microphysical species by fallspeed, and 3) different rates of the parameterized microphysical conversion processes. The results confirm previous studies on the thermodynamic effect of melting, but they also show that the other factors, namely, fallspeed and microphysical conversion rates, are important in determining model vertical velocity structure and evolution. Statistical summaries of updrafts and downdrafts show distinct increases in the intensity and horizontal scale of downdrafts near the melting level when parameterized snow is included. Model storms without snow show a greater percentage of convective-scale updrafts and downdrafts; they intensify more slowly but ultimately become stronger than those that have larger scale vertical velocity structures.

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Stephen J. Lord and Akio Arakawa

Abstract

The closure assumption of the Arakawa-Schubert (1974) cumulus parameterization takes the form of a balance between the generation of moist convective instability by large-scale processes and its destruction by clouds. This assumption can be justified by consideration of the kinetic energy budget of a cumulus subensemble. First, the kinetic energy generation and dissipation per unit cloud-base mass flux should approximately balance over time scales of the order of the large-scale processes. Second, the dissipation per unit cloud-base mass flux and, therefore, the kinetic energy generation per unit cloud-base mass flux (the cloud-work function) for a given subensemble should not depend substantially on the large-scale conditions. The cloud-work function quasi-equilibrium follows consequently and the unknown cloud-base mass flux is determined by an integral equation.

Observational evidence for the cloud-work function quasi-equilibrium is presented. Cloud-work functions are calculated from a variety of data sets in the tropics and subtropics including the GATE, AMTEX, VIMHEX and composited typhoon data. The results show that the cloud-work functions fall into a well-defined narrow range for each subensemble although the thermodynamical vertical structures for each data set are quite different.

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Stephen J. Lord, Hugh E. Willoughby, and Jacqueline M. Piotrowicz

Abstract

Results of an axisymmetric, nonhydrostatic hurricane model are analyzed with emphasis on the role of a parameterized ice-phase microphysics Inclusion of ice processes produces dramatic differences in the structure and evolution of the simulated hurricane vortex. Mesoscale convective features are wore plentiful with ice, and the simulated vortex grows more slowly.

Time and space-averaged budgets of key model varibles show that cooling due to melting ice particles can initiate and maintain model downdrafts on a horizontal scale of tens of kilometers. This scale depends critically on both the horizontal advection of the parameterized snow particles detrained from the tops of convective updrafts and the mean fall speed of the particles toward the melting level. In situ0 production of snow particles results from a wide variety of parameterized microphysical processes and is significant factor in maintaining upper-level snow concentration These processes are strongly height-dependent.

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Stephen J. Lord, Winston C. Chao, and Akio Arakawa

Abstract

An application of the Arakawa-Schubert (1974) cumulus parameterization to a prognostic model of the large-scale atmospheric circulations is presented. The cloud subensemble thermodynamical properties are determined from the conservation of mass, moist static energy and total water (vapor, suspended liquid water and precipitation). Algorithms for calculating the large-scale forcing and the mass flux kernel are presented. Several methods for solving the discrete version of the integral equation for the cumulus mass flux are discussed. Equations describing the cumulus feedback on the large-scale thermodynamical fields are presented.

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Huge E. Willoughby, Han-Liang Jin, Stephen J. Lord, and Jacqueline M. Piotrowicz

Abstract

This paper reports numerical simulations of the hurricane vortex by an axisymmetric, nonhydrostatic numerical model with 2 km maximum horizontal resolution. Moist convection is modeled explicitly using two different microphysical parameterizations. The first simulates liquid water processes only, whereas the second includes ice processes as well.

Although concentric rings of convection associated with local maxima of the tangential wind form in both versions of the model, they are much more common when ice processes are included. As they contract about the vortex center, the outer ones supplant the inner. Their contraction follows the mechanism suggested by balanced-vortex models. Some of the rings appear to form through symmetric instability of the vortex, and others—particularly when ice processes are included—through interactions between precipitation-induced downdrafts and the boundary layer. Both the rings’ evolution and the detailed structure of the vortex core are similar to recent aircraft and radar observations. Among the realistic features are: outward slope of the eyewall updraft and tangential wind maximum; relative location of the updraft, wind maximum, and precipitation maximum; stratiform precipitation and mesoscale downdrafts outside the eye; and midlevel radial inflow.

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